1. Field of the Invention
The present invention relates to a process for producing an optical glass according to an ion exchange method. The optical glass obtained according to the present invention is largely divided into a uniform reflactive index type glass and a gradient refractive index type glass. The former glass is used mainly as an optical glass in the fields of optics and optoelectronics, and the latter glass is used as a gradient index lens mainly for optical coupling between optical fibers or between a light source and an optical fiber, or as an microlens array.
2. Description of the Prior Art
The conventional processes for producing such a uniform refractive index type glass or such a gradient refractive index type glass include, for example, a process according to an ion exchange method which is described in Japanese Patent Application Kokai (Laid-Open) No. 222943/1986.
In the process, a glass containing Na.sup.+, K.sup.+, etc. in large amounts is immersed in a molten salt containing ions giving a higher refractive index, such as Ag.sup.+, Tl.sup.+, Li.sup.+ and the like to subject the ions contained in the glass (i.e. Na.sup.+, K.sup.+, etc.) to sufficient ion exchange with the ions giving a higher refractive index (i.e. Ag.sup.+, Tl.sup.+, Li.sup.+, etc.) and thereby to produce a uniform refractive index type glass in which the concentration of the ions giving a higher refractive index is uniform throughout the entire ranges from the center of the glass to its outer surface and whose refractive index is uniform throughout said ranges, and then this glass is subjected to ion exchange in a molten salt containing Na.sup.+, K.sup.+, etc. to produce a gradient refractive index type glass.
However, in producing a uniform refractive index type glass, the above process has a drawback in that only one kind of uniform refractive index type glass having a particular refractive index is obtained from one type of starting glass as long as the type of the molten salt remains same, because the ion exchange between the ions contained in the starting glass and the ions giving a higher refractive index is conducted until the concentrations of these ios in glass reach an equilibrium.
Various glasses of uniform refractive index type, ranging in refractive index from 1.46 to 1.98 and in Abbe's number .nu.d from 20 to 91 are required even now for designing of diversified optical products. Therefore, in order to produce, according to the conventional process, a uniform refractive index type glass having a desired refractive index and other good optical properties so as to meet the intended applications(s), it has been necessary to prepare in advance according to the melting method, a number of starting glasses each having a different and particular composition or ion concentration as to enable production of said various glasses.
Also in producing a gradient refractive index type lens, the above process has a drawback in that only one kind of gradient refractive index type lens having a particular numerical aperture is obtained because a lens satisfying optical properties (e.g. aberration) can be obtained only in an optimum ion exchange time. A preferable numerical aperture is said to be at least 0.6 when a gradient refractive index type lens is used for optical coupling between a LD light source and an optical fiber, 0.2 to 0.3 when the lens is used for optical coupling between optical fibers, and 0.05 to 0.2 when the lens is used as a microlens array for copying machine. Therefore, in order to produce, according to the conventional process, lenses each having any desired numerical aperture so as to be used in any particular applications, it has been necessary to prepare in advance according to the melting method, a number of starting glasses each having a different and particular composition or ion composition as to enable production of said various lenses.
Hence, the first object of the present invention is to provide a process for producing from a same starting glass, a number of uniform refractive index type glasses each having any desired and different refractive index and good optical properties.
The second object of the present invention is to produce from a same starting glass, a number of gradient refractive index type glasses each having any desired and different refractive index distribution and good optical properties.
Other objects of this invention will become apparent from the following description and the accompanying drawings.